1. Field of the Invention
The present invention relates to method and apparatus for controlling a pulse width modulated (PWM) converter. The term converter herein used means a power converter from A.C. to D.C. and a power converter from D.C. to A.C. (usually called an inverter).
2. Description of the Prior Art
A pulse width modulated (PWM) converter has been well known. For example, "Toshiba Review" Vol. 35, No. 12, pages 1054-1056, shows a PWM inverter using power transistors and "Hitachi Review" Vol. 60, No. 6, pages 29-34, shows an inverter using gate turn-off (GTO) thyristors. A semiconductor device having a self-turn-off function such as GTO thyristor can be turned on by a short duration on-gate signal pulse if a current flowing therethrough is above a holding current level and the conduction state is maintained after the removal of the gate bias. In the past, however, an on-gate signal is supplied continuously during a period from the time of turn-on to the time of turn-off even under the holding condition. This results in a power loss and a required increase of power capacity in a gate circuit. It is known that an assembly of devices including a switching device (e.g. GTO thyrister) and devices in a control apparatus for generating a firing signal exhibit a large power loss when they are switched on and off but that once they are conducted they require only a small power loss. During the conduction period, however, they are normally powered by a power supply. As a result, the apparatus is of large size and dissipates extra power.